This invention relates to a process for the treatment, at the oil field, of highly viscous and dense heavy oils, to produce an easily transportable mixture of hydrocarbons of reduced viscosity. The invention also has for an object to facilitate the desalting of heavy oils in the field by decreasing their viscosity and their density by addition of a hydrocarbon cut obtained at the oil field.
Today, one of the limiting factors in the development of the production of heavy oils of high density (d.sub.4.sup.15 higher than 0.98, i.e. an API degree lower than 12.9) and very high kinematic viscosity (.eta..sub.100.degree. C. &gt;50 cSt (mm.sup.2 /s), .eta..sub.37.8.degree. C. &gt;1940 cSt (mm.sup.2 /s)), is that of their transportation from the production zones to the refining locations.
As a matter of fact, although accurate specifications do not exist for the transportation by pipe-line, in accordance with the usually admitted rules, the kinematic viscosity must not exceed 120 cSt (mm.sup.2 /s) at 20.degree. C. (a value retained for the South-European pipe-line), or must be lower than 400 cSt (mm.sup.2 /s) at 37.8.degree. C., which is the admitted value in Venezuela, a country having substantial resources of heavy oils. It is observed that the viscosity at 37.8.degree. C. of the heavy oils from Venezuela is mostly higher than 10,000 cSt (mm.sup.2 /s); examples thereof are Boscan: 18,600 cSt (mm.sup.2 /s), Laguna Once: 10,700 cSt (mm.sup.2 /s) and Cerro Negro (35 800 cSt): these values are thus substantially higher than the above limits and these heavy oils cannot be transported as such through pipe-lines.
Various attempts have been made to solve this problem of transporting highly viscous oils; examples thereof are:
1. Heating of pipelines; this solution is very costly, especially where the production fields are far from the refineries. PA0 2. Transportation of heavy oils as oil-in-water stable emulsions; this technology, also very costly, since it results in the transportation of about 30 to 40% of water, suffers from additional disadvantages, such as the necessary demulsification after delivery from the pipeline. PA0 3. Dilution of heavy oils, either with a light or middle crude oil or with middle distillates of low viscosity. In both cases, the problem of transporting dilution agents from their production area to heavy oil fields arises, with the necessary consequence of an additional cost.
As concerns the desalting of heavy and viscous crude oils in the field, other problems arise; the electrostatic desalters which can be used for desalting these oils are adapted to operate efficiently at dynamic viscosity levels not exceeding 4 to 5 mPa.s at the operating temperature, generally from 60.degree. to 150.degree. C. A great number of heavy and viscous oils exceed this limit to a very large extent; thus, Boscan crude oil has, at 150.degree. C., a dynamic viscosity of 32 mPa.s. On the other hand, the settling rate of both oil and water phases in the desalter is proportional to the difference in density of the two phases. This difference is very small for most of the heavy crude oils. Thus, at 150.degree. C., the difference in density between Boscan crude oil and water is only 0.09.
These problems of heavy oils desalting may be solved by the above solution (3), but it is apparent that there is place on the market for a process for treating heavy and viscous oils in the field, providing in situ a hydrocarbon cut of low viscosity and middle density, usable as dilution agent for the crude oil in order to permit its desalting in the field as well as its transportation.
Among the proposals made up to now for this type of process, there can be mentioned U.S. Pat. No. 3,474,596 (U.S. Pat. No. Re. 27,309), as well as French Pat. No. 2,489,835. The process disclosed in these two patents is crude oil visbreaking in the absence of hydrogen.
French Pat. No. 2,489,835 discloses a process for the pretreatment of predesalted heavy crude oils, comprising visbreaking, followed with distillation for separating two fractions: a synthetic transportable crude oil, obtained with a yield of 66 to 75%, and a non-transportable residue.
U.S. Pat. No. 3,474,596 (U.S. Pat. No. Re. 27,309) concerns a process wherein visbreaking is performed on a portion of the crude charge, after desalting, and the visbroken product is recycled in admixture with the crude oil before desalting.
This process, applied to oils having an API degree from 14 to 24 (i.e. densities d.sub.4.sup.15 from 0.972 to 0.910) would result in the production of a mixture of transportable hydrocarbons, and in the desalting in the field of the crude oils.
However, it has been observed that the visbreaking treatments effected in the absence of hydrogen lead to hydrocarbon mixtures whose heavier fractions have a poor stability, which can result in settling of a portion of said fractions and more particularly of asphaltenes, mainly during the transportation or the storage thereof.